The use of microstrip in the field of microwave circuit design is fairly well known and understood in the art. Microstrip consists of a single dielectric substrate with a conductive ground plane on one face of the substrate and a metallized layer on the other face. A microstrip antenna is typically a rectangular patch of metal etched on the metallized coating side. A signal is applied to the antenna via a connector at a feed point on the antenna, normally at one edge of the patch.
As is also well known in the art, when two sections or components of different impedances are connected together, an impedance transformer is invariably required to ensure maximum power transfer from one section to another. In the case of a microwave antenna, it is necessary to match the input impedance of the antenna to that of the antenna feed line, in order to maximize power transfer from the feed line to the antenna. This matching is normally performed by a section of metal extending from the antenna patch and the end of which is connected to the feed connector. A typical matching system is shown in Canadian patent 1,097,428, having a thin line of metal, which for this specific example has a width of 0.2 inches and a length of about 11/2 inches.
Conventional edge-fed microstrip patches have a very high input impedance. The input impedance (z) of a line in microstrip is approximately proportional to the width (W) of the line and inversely proportional to the thickness or height (h) of the substrate (z approximately proportional to (W/h)). It may be seen then that for a very narrow substrate, that is a small value of h, the width of the line would have to be also very small in order to transform to a useable input impedance, i.e. an impedance lower than that of a patch antenna. This situation is adequate as long as the RF power into the line is relatively small. However, in high power applications the width of the line becomes a limitation and would tend to burn up.
A further problem in matching Occurs when a relatively thick substrate is used. In this situation, as is well known in the art, the impedance matching section would have to be relatively wide. This relatively wide section affects the radiation pattern of the patch element. Consequently, it is necessary to use a thin substrate on a separate layer in order to transform the impedance. However this transformation to a thin substrate results in a narrow matching section, which once again is limited in its power handling. a further problem is that this section is placed out of the plane of the patch element. Various ways of achieving this are well known in the art. In modern applications of antennas, such as in cellular base stations and the like, space is extremely limited. Therefore, placing a matching section out of the plane of the antenna is not desirable.
It is an object of this invention to provide a radiating element that has a low input impedance and eliminates the need for a narrow matching transformer, and thus allowing higher RF power to be supplied to the antenna, and without interfering substantially with the radiation pattern of the antenna, while also achieving a relatively easy to manufacture and compact design.